This study was designed to explore risk factors for breast cancer with emphasis on the detection of clinical markers of the hormonal imbalance during the perimenarche. Three hundred and thirty women diagnosed with breast cancer and 346 population controls were identified and interviewed in Girona, Spain between 1986 and 89. Cases were more likely than controls to have had long menstrual periods in the first 5 years after menarche [odds ratio (OR)=3.0], to experience menopause at a late age (OR=1.5) and to report acne during adolescence (OR=1.6). Family history of breast cancer was associated with an increased risk (OR=2.3). Cases reported a lower use of drug treatments for anxiety and sleep disorders than controls. Moderate alcohol drinkers and smokers were at lower risk for breast cancer. No statistically significant association with breast cancer was observed for number of children, age at last pregnancy, oral contraceptive use, hormonal treatment after menopause and weight perception during the teenage years. Hormonal changes in the years following menarche may be relevant to breast cancer risk. The roles of menstrual period length and acne during adolescence should be further explored.